1. Field of the Invention
The present invention relates to a dual-band surface acoustic wave filter and a composite high-frequency component. Particularly, the present invention relates to a dual-band surface acoustic wave filter preferably for use in a composite high-frequency component including a high-frequency switch, and a composite high-frequency component including such a dual-band surface acoustic wave filter.
2. Description of the Related Art
In recent years, cellular phones supporting a plurality of communication methods having different frequency bands have been popularized. In such a cellular phone, in order to transmit and receive transmission and reception signals in the plurality of communication methods using a single antenna, a switch module capable of switching among a plurality of transmission and reception signals is used.
For example, Japanese Unexamined Patent Application Publication No. 2007-266840 described below describes a switch module 100 shown in FIG. 11, as an example of such a switch module.
As shown in FIG. 11, the switch module 100 includes a circuit board 101 composed of a plurality of laminated dielectric layers. On a front surface of the circuit board 101, a high-frequency switch 102, a plurality of surface acoustic wave filters 103a to 103c, a plurality of chip capacitors 104a to 104d, and a chip inductor 104e are mounted. It should be noted that among the plurality of surface acoustic wave filters 103a to 103c, each of the surface acoustic wave filters 103a and 103c is a single-band surface acoustic wave filter including one surface acoustic wave filter portion. Meanwhile, the surface acoustic wave filter 103b is a dual-band surface acoustic wave filter including two surface acoustic wave filter portions.
Although not shown, each of output terminals of the surface acoustic wave filters 103a to 103c is connected via a via-hole electrode continuous in a lamination direction of the circuit board 101, to an external terminal formed on a back surface of the circuit board 101. Thus, the external terminals of the circuit board 101 are located almost directly below the output terminals of the surface acoustic wave filters 103a to 103c, respectively, and hence an output side wiring that connects each output terminal to each external terminal is shortened and two output side wirings next to each other do not overlap each other in the lamination direction and are not close to each other. Therefore, a parasitic impedance and a parasitic capacitance are unlikely to occur.
Further, in this case, near an edge portion of the back surface of the circuit board 101 on an X1 side in an x direction, a plurality of external terminals can be arranged along a y direction perpendicular to the x direction. Thus, wirings that connect the external terminals of the circuit board 101 to an RF-IC in which a part of an RF circuit at a stage subsequent to the switch module 100 is integrated can be shortened.
However, in the switch module 100 shown in FIG. 11, on the front surface of the circuit board 101, the plurality of surface acoustic wave filters 103a to 103c are arranged near the edge portion on the x1 side and along the y direction, and the high-frequency switch 102 is arranged near a center of an edge portion on an x2 side. Thus, the length dimension, in the y direction, of the space in which the plurality of surface acoustic wave filters 103a to 103c are arranged tends to be larger than the length dimension, in the y direction, of the high-frequency switch 102. Therefore, there is a problem that dead spaces occur on both sides of the high-frequency switch 102 in the y direction and the switch module 100 is increased in size.
Thus, for example, it is also considered that as shown in FIG. 12, two single-band surface acoustic wave filters 103a and 103b and two single-band surface acoustic wave filters 103c and 103d are arranged on both sides, respectively, of the high-frequency switch 102 in the y direction. In this case, a dead space on the circuit board 101 can be smaller than that when the surface acoustic wave filters 103a to 103d are arranged linearly. Thus, the switch module can be reduced in size.
In the switch module shown in FIG. 12, unbalanced input terminals 105a to 105d of the surface acoustic wave filters 103a to 103d are connected to the high-frequency switch 102. Thus, as shown in FIG. 12, the unbalanced input terminals 105a to 105d are preferably arranged near the high-frequency switch 102. When so configured, first and second balanced output terminals 106a to 106d and 107a to 107d of the surface acoustic wave filters 103a to 103d are arranged on the opposite sides of the surface acoustic wave filters 103a to 103d in the y direction with respect to the high-frequency switch 102.
Here, when external terminals are provided directly below the output terminals of the surface acoustic wave filters 103a to 103d similarly to the case shown in FIG. 11, a plurality of the external terminals are separately arranged near a y1-side short side and a y2-side short side of the circuit board 101 in the case shown in FIG. 12. Thus, at least either of the wirings drawn from the y1-side external terminals and the wirings drawn from the y2 side bypass and are connected to the RF-IC. As a result, the wirings are longer than existing ones, and hence the area of the board on which the high-frequency switch 102 and the RF-IC are mounted increases.
Thus, in the case shown in FIG. 12, it is necessary to arrange a plurality of external terminals 108a to 108h near the edge portion of the circuit board 101 on the x1 side and along the y direction and to electrically connect these external terminals 108a to 108h to the first and second balanced output terminals 106a to 106d and 107a to 107d of the surface acoustic wave filters 103a to 103d via wirings formed on the front surface of the circuit board 101 and within the circuit board 101.
However, as a result of an experiment, the inventor of the present invention has discovered that the filter characteristics of the surface acoustic wave filters of the switch module deteriorate when the arrangement configuration as shown in FIG. 12 is used. In other words, when the arrangement configuration as shown in FIG. 12 is used, output side wirings 110a to 110h that electrically connect the plurality of external terminals 108a to 108h to the first and second balanced output terminals 106a to 106d and 107a to 107d, and input side wirings 111a to 111d that electrically connect the high-frequency switch 102 to the unbalanced input terminal 105a to 105d, intersect each other in a lamination direction of the circuit board 101 and are close to each other as shown in FIG. 13. As a result, it was discovered that parasitic components occur between the output side wirings 110a to 110h and the input side wirings 111a to 111d and thus isolation between input and output and balance of a balanced output signal deteriorate.